A detector of this general type is known for example from U.S. Pat. No. D652,330 S. In this detector the support rail is configured as a circular rod which is supported by two clamps which are arranged at a battery compartment that is pivotably connected with the rear tube end of the rear tube element. The support rail extends over an entire length of the battery compartment. The support rail is divided by one of the clamps into a forward rail section and a rear rail section. The rear rail section oriented away from the support tube supports the support slide with the arm shell. Another support slide is arranged at the forward rail section oriented towards the support tube, wherein the other support slide supports the handle element wherein the handle element is pivotably supported in a longitudinal direction of the support tube at the second support slide. The two support slides are supported with their flat bottom sides on the battery compartment, so that a lateral movement of the two slides in a circumferential direction of the cylinder shaped support rail is excluded. The two support slides and the battery compartment include locking elements that are associated with each other wherein the slides are disengageably attached at predetermined locations of the support rail using the locking elements. The two arm support bars that are pivotably arranged at the rear support slide are configured in two components. A first bar element is connected with the support slide through a pivot link and the other second arm bar element is connected with the first bar element through a hinge. The two bar elements are thus freely moveable orthogonal to the support tube and can be pivoted away from one another and towards each other at will. In order to fixate the four bar elements forming the arm shell a tightening belt is provided which is supported by recesses of the two arm support bars and of the slide. Using the tightening belt the two arm support bars can be brought into contact with a lower arm of a user of the detector. The battery compartment on which the support slide is arranged is linked at the rear tube end of the rear tube element of the support tube. The known detector can be moved from a longitudinally extended operating position for detecting objects into a folded space saving idle position for transporting or storage and vice versa. In the compact idle position the tube elements of the telescoping support tube are slid into each other and the support tube is thus shortened. In this position namely the measuring probe and the battery compartment which are respectively connected through a pivot link with the forward or rear tube element of the support tube are pivoted against the support tube and extend in parallel and in contact with the support tube. In the operating position of the detector the arm support bars extend laterally from the battery compartment and the arm support elements are folded into contact with the battery compartment in an idle position of the detector.
It is a disadvantage of this known detector that the two arm support elements are freely moveable in the idle position and also in the operating position and have to be brought into the respective position by hand and have to be supported in these positions using the tightening belt. This is complicated and uncomfortable for a user. It is furthermore disadvantageous that the support rail has a circular cross section which requires supporting the associated support slide on the battery housing so that the support slide does not rotate in the circumferential direction of the support slide.